General surface corrosion of metals by chemicals dissolved in an electrolyte is a common problem in the operation of industrial processes which utilize amines or alkanol amines as reactants, products, solvents or the like and is treated in a variety of ways. For example, U.S. Pat. No. 4,071,470 discloses a method of inhibiting the corrosion of metals in contact with an acid-gas absorbing medium by use of the reaction product of (1) certain copper materials, and (2) sulfur or a sulfide and an oxidizing agent, and (3) a monoalkanol amine prepared under anhydrous conditions. U.S. Pat. No. 4,096,085 discloses an aqueous organic amine acid gas scrubbing system having incorporated therein (1) certain amines, (2) a copper material, and (3) sulfur or a sulfur yielding compound. U.S. Pat. No. 4,088,735 discloses the purification of gases from the gasification of fossil fuels in three scrubbing stages in which the second stage uses a high-boiling organic solvent which is miscible with water (e.g., pyrrolidones, polyglycol ethers, tetrahydrothiophene-1,1-dioxide, butyrolactone, morpholine and N-methyl-epsilon-caprolactam and equivalent kinds of solvents) and to which sulfur has been added to promote the removal of methyl mercaptan by forming higher boiling disulfides.
Generally, stress corrosion cracking of metal weldments is a different problem than the above-described general corrosion of metal surfaces and involves the accelerated localized corrosion of a metal under a continuing stress in which the effects are greater than those of either the corrosive environment or the stress alone. The higher the strength of a given material, the greater its tendency for stress corrosion cracking and highly alloyed materials generally exhibit greater susceptibility toward this type of attack. A period of "initiation" is required in which localized corrosion occurs that may take the form of pits or notches. Cracking initiation follows the induction period if conditions are appropriate for inducing cracking. As a metal yields to corrosion, the surface of the metal is broken up preventing the formation of a protective oxide or sulfide film and thereby presenting a continued supply of unprotected metal to the corrosive environment. Thus, stress corrosion cracking is not the same as common corrosion and occurs in situation of low general corrosion but results in deep cracking of the metal often in the heat affected zone of the weldments.
Metal stress, where stress corrosion cracking can occur, is introduced into metal weldments by the very nature of the welding process in which the heat of welding causes the localized properties of the metal around the weld to be changed from the remainder of the metal in the equipment. One way of reducing stress corrosion cracking of equipment having metal weldments is to subject the equipment after welding to a stress-relieving treatment in which the entire piece of equipment is heated to provide a uniform metal structure. However, this stress-relieving treatment is expensive and time-consuming. Thus, equipment in gas treating units can often be made of non-stress relieved metal weldments.
Although alkanol amine treating facilities have been used for hydrogen sulfide and/or carbon dioxide acid gas removal since the early 1950's, stress corrosion cracking of metal weldments in these units was not a major concern until recently when application of the wet fluorescent magnetic particle technique revealed extensive cracking problems in equipment. A National Association of Corrosion Engineer's (NACE) survey in 1985/1986 confirmed that the amine stress corrosion cracking of metal weldments represented by tight, branched, primarily intergranular cracks is a relatively common failure mode in non-stress relieved carbon steel equipment in monoethanol amine plants and, to a lesser degree, in diethanol amine or methyldiethanol amine gas treating facilities. Temperatures at which this type of cracking has been observed cover the entire range encountered in alkanol amine acid gas treating plants, namely from about 100.degree. F. to about 250.degree. F.
Applicant has discovered that stress corrosion cracking of metal weldments in the presence of alkanol amines proceeds by a novel mechanism caused by a combination of carbon dioxide, carbonates, certain weak sulfiding agents (e.g., sulfur containing salt compounds), cyanides and chlorides present as contaminants in the aqueous alkanol amine solution. The following mechanism is proposed: The carbonats in solution form a protective carbonate/magnetite film on the metal surface, whose potential is shifted into the cracking range by carbon dioxide in the atmosphere. The integrity of the carbonate film is affected by sulfide inclusions which form in the presence of weak sulfiding agents, such as thiosulfate ions. Contamination by chloride and/or cyanide appear to rupture the magnetite/sulfide film locally so that stress corrosion cracking can proceed. The alkanol amine solvent solution is believed to only provide the appropriate pH and, thus, the proper carbonate/bicarbonate ratio for stress corrosion cracking to occur. Standard slow strain rate tensile test results indicate that failures in alkanol amine stress corrosion cracking of metal weldments are induced by exposure of non-stress relieved carbon steel to an alkanol amine solvent solution of complex composition due to the above described contaminants. Cracking proceeds basically in two stages. Conspicuous, blunt cracks--better described as fissures--first form caused by the activity of carbon dioxide and carbonates in alkanol amine solvents and may be classified as carbonate/bicarbonate type cracks. In the second stage, cracks, propagating in an intergranular mode, originate at the bottom of the blunt fissures caused by weak concentrations of sulfiding impurities (e.g., sulfur containing salt compounds) in the presence of chloride and/or cyanide. Stress cracking patterns developing in the standard slow strain rate tensile test metal specimens exposed to fresh alkanol amine solvent solution with the above contaminant components added are quite similar to those in metal specimens exposed to carbon dioxide saturated, lean alkanol amine plant solvent. When the weak concentrations of sulfiding salt compounds are absent, fissuring merges into general corrosion at high alkanol amine concentrations (about 50%). In the presence of weak concentrations of sulfiding sulfur containing salt compounds, but in the absence of chloride, cracks become relatively shallow, blunt and primarily transgranular. However, in the presence of both weak concentrations of sulfiding salt (thiocyanate) compounds and chloride, cracking is primarily intergranular (characteristic of stress corrosion cracking of metal weldments) in solvent solutions containing 15% and 25% monoethanol amine and strictly transgranular in the 50% monoethanol amine solvents.
Accordingly, it is desirable to reduce or prevent the stress corrosion cracking of metal weldments of industrial equipment which comes in contact with corrosive chemicals, such as contaminated alkanol amine reactants, products and solvents.